When climate camp protesters descended on the site of the Kingsnorth power station for a week-long summer demonstration, the scale of the police operation to cope with them was enormous.

Police were accused of using aggressive tactics, confiscating everything from toilet rolls and board games to generators and hammers. But ministers justified what they called the "proportionate" £5.9m cost of the operation, pointing out that 70 officers had been injured in the course of their duties.

But data obtained under the Freedom of Information Act puts a rather different slant on the nature of those injuries, disclosing that not one was sustained in clashes with demonstrators.

Papers acquired by the Liberal Democrats via Freedom of Information requests show that the 1,500 officers policing the Kingsnorth climate camp near the Medway estuary in Kent, suffered only 12 reportable injuries during the protest during August.

The Home Office has now admitted that the protesters had not been responsible for any injuries. In a three-line written answer to a parliamentary question, the Home Office minister Vernon Coaker wrote to the Lib Dem justice spokesman, David Howarth, saying: "Kent police have informed the Home Office that there were no recorded injuries sustained as a result of direct contact with the protesters."

Only four of the 12 reportable injuries involved any contact with protesters at all and all were at the lowest level of seriousness with no further action taken.

The other injuries reported included "stung on finger by possible wasp"; "officer injured sitting in car"; and "officer succumbed to sun and heat". One officer cut his arm on a fence when climbing over it, another cut his finger while mending a car, and one "used leg to open door and next day had pain in lower back".

A separate breakdown of the 33 patients treated by the police tactical medicine unit at the climate camp shows that three officers had succumbed to heat exhaustion, three had toothache, six were bitten by insects, and others had diarrhoea, had cut their finger or had headaches.

Coaker claimed in a parliamentary debate in September that the police had acted "appropriately and proportionately", despite hundreds of complaints over unnecessarily heavy policing and calls for an investigation of police conduct by MPs, MEPs, councillors and members of the public.

Norman Baker, the Lib Dem MP for Lewes, who had called previously for an investigation of police tactics, said: "I personally witnessed unnecessarily aggressive policing, unprovoked violence against peaceful protesters, an extraordinary number of police on site, and tactics such as confiscating toilet rolls, board games and clown costumes from what I saw to be peaceful demonstrators."

The list of items deemed potentially dangerous by police and seized from protesters included glue, marker pens, board games, cushions, carpet, wood, paint, and scissors as well as bicycle locks which could have been used to lock protesters to fences. Police also seized anything that could have been used to set up camp, including spades and duct tape, generators and hammers and nails.

Howarth said: "That the minister could defend as 'proportionate' a £5.9m policing operation in which there was not a single injury to police officers caused by the protesters beggars belief. The threat posed by environmental direct action is being systematically overblown by both the government and the police.

"I hope the government and the police will now stop trying to portray peaceful protesters as somehow equivalent to terrorists or violent extremists. In light of this new evidence, one has to ask, were climate campers so heavily policed because they posed any genuine threat of violence, or because they posed a challenge to government policy?"

Nick Thorpe, a spokesman for the climate camp, said: "Policing of peaceful protest has become increasingly heavy-handed. We saw thousands of officers swarming around a legal camp in a colossal waste of public money. The police and the government claimed there was a 'violent minority' of protesters but this Home Office admission reveals this as a complete fiction."

Coal controversy

Kingsnorth, a power station built 45 years ago on the Medway estuary in Kent can burn oil as well as coal. Two years ago its owner, the energy company E.ON UK, announced plans to build two "cleaner coal units" there. The new plants are the focus of protest for climate-change activists who believe the development will discharge carbon dioxide and other greenhouse gases at too high a level. The Kingsnorth development will involve the first purpose-built, coal-fired power stations in the UK since completion of the Drax station in Yorkshire in 1986. E.ON claims the majority of the local population favours the project, and says a carbon capture and storage system "could eventually see 90% of its emissions captured and stored underground". The company insists its port facilities on the Hoo peninsula and its own fleet of colliers will let it easily import low-sulphur coal. The new plants are meant to be operational in 2012. Of Kingsnorth, Greenpeace says: "[It will] emit the same amount of CO2 as the 30 least polluting countries in the world combined, and destroy any chance we have of persuading China and India to stop building coal plants." Owen Bowcott